The present invention generally relates to a system and method for improving the navigation accuracy of an electromagnetic navigation system for use with medical applications. Particularly, the present invention relates to a system and method for improving the calibration of a fluoroscope camera by compensating for the S-distortion.
Electromagnetic type navigation systems are useful in numerous applications. One application of particular use is in medical applications, and more specifically, image guided surgery. Typical image guided surgical systems acquire a set of images of an operative region of a patient's body and track a surgical tool or instrument in relation to one or more sets of coordinates. At the present time, such systems have been developed or proposed for a number of surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac or other interventional radiological procedures and biopsies. Such procedures may also involve preoperative or intraoperative x-ray images being taken to correct the position or otherwise navigate a tool or instrument involved in the procedure in relation to anatomical features of interest. For example, such tracking may be useful for the placement of an elongated probe, radiation needle, fastener or other article in tissue or bone that is internal or is otherwise positioned so that it is difficult to view directly.
An electromagnetic tracking system may be used in conjunction with an x-ray system. For example, an electromagnetic tracking system may be used in conjunction with a C-arm fluoroscope. The C-arm fluoroscope may utilize an x-ray source at one end of the C-arm and an x-ray detector, or camera, at the other end of the C-arm. The patient may be placed between the x-ray source and the x-ray detector. X-rays may pass from the x-ray source, through the patient, to the x-ray detector where an image is captured. The electromagnetic tracking system may generate an electromagnetic field between the ends of the C-arm and penetrate the body with minimal attenuation or change so tracking may continue during a surgical procedure.
Part of the X-ray detector may include an X-ray image intensifier device (IID). The function of the IID in the fluoroscopic imaging system is to convert the x-ray spectrum transmitted through the patient into a highly visible image. The image is produced by converting the x-ray photons into light photons at the image intensifier input phosphor, converting the visible light photons into electrons at the photocathode, accelerating and focusing the electrons through use of electrodes, and finally, converting the electrons back into visible light at the output phosphor. The intensity of the final image is several thousand times brighter than the initial image created at the input phosphor. The IID allows for lower x-ray doses to be used on patients by magnifying the intensity produced in the output image, allowing the viewer to more easily see the structure of the object being imaged.
In general, there are a variety of imperfections in IIDs, including pincushion distortion and S-distortion. Pincushion distortion is at least partially caused by the mapping of electrons from the curved input surface to a flat output screen. The mapping from a curved surface to a flat surface may cause larger magnification at the image periphery as compared to the center. S-distortion associated with the IIDs is at least partially caused by the magnetic field effect of the earth on the paths of the moving electrons within the IID. The resulting distortion usually has a characteristic “S” shape. For example, electrons within the IID move in paths along designated lines of flux. External electromagnetic sources, such as the earth's electromagnetic field, affect electron paths at the perimeter of the image intensifier more so than those nearer the center. This characteristic causes the image in a fluoroscopic system to distort with an S shape. Since the magnitude of the earth's magnetic field varies as the IID's position is changed, the S-distortion pattern may vary.
One technique that has been used to address the variances of the S-distortion pattern is to arrange the mu-metal shield to reduce the residual earth magnetic fields inside the IID tube. Such an arrangement may include adding an active coil to the IID to compensate for the earth's magnetic field or introducing a distortion sensing mechanism in conjunction with the active compensation coil to dynamically correct the actual distortion. These techniques are generally not sufficient for use with 3D imaging or navigation purposes.
Another technique that has been used to address the variances of the S-distortion pattern is to perform calibration. Calibration may be performed off-line or online. The off-line calibration may be used for the fixed room or mobile C-arm with repeatable motion control. The disadvantage of off-line calibration is that it the C-arm is generally non-mobile. The online calibration use a calibration target embedded with fiducial markers. One disadvantage of the on-line calibration technique is a potential high sensitivity to miss-detection of the fiducial shadow that may be obscured by patient anatomy or the surgical table.
Accordingly, a system and method is needed to better address the variances of the S-distortion. Such a system and method may improve navigation system accuracy as well as reduce the camera calibration re-projection error.